Summary: Fundamentals of Ethernet LANs (Chapter 2)

Chapter Overview

This chapter provides a foundational understanding of Ethernet LANs, focusing on their physical and data-link layers. It explores Ethernet standards, cabling types, frame formats, and operational mechanisms, critical for CCNA exam preparation and real-world network deployments.

Key Concepts

• LAN Types: Differentiation between wired (Ethernet) and wireless LANs.
• Ethernet Standards: IEEE 802.3 defines Ethernet protocols and technologies.
• Duplex Modes: Full-duplex for simultaneous send/receive; half-duplex for legacy hubs.
• MAC Addressing: Core of Ethernet frame delivery (unicast, multicast, broadcast).

Detailed Explanations

LAN Types

1. SOHO Networks:
   • Typically use consumer-grade routers combining Ethernet switching and wireless functionality.
   • Devices include PCs, printers, and wireless clients connecting via a single switch.
2. Enterprise Networks:
   • Feature distributed switching infrastructure across multiple floors or buildings.
   • Use structured cabling systems with access, distribution, and core layers.

Ethernet Standards

1. Physical Media:
   • UTP (Unshielded Twisted Pair): Economical, widely used, max length 100m.
   • Multimode Fiber (MM): Costlier, suitable for 500m with LED transmitters.
   • Single-mode Fiber (SM): Supports tens of kilometers with laser-based transmitters.
2. Data Rates:
   • 10BASE-T: 10 Mbps over UTP.
   • 100BASE-T: 100 Mbps over UTP.
   • 1000BASE-T: 1 Gbps over UTP.
   • 10GBASE-T: 10 Gbps over UTP.
3. IEEE Naming Conventions:
   • Prefix denotes speed (e.g., 100BASE- for 100 Mbps).
   • Suffix indicates medium (e.g., T for twisted pair, X for fiber optics).

Ethernet Frame Format

1. Header Fields:
   • Preamble (7 bytes): Synchronization.
   • SFD (1 byte): Start Frame Delimiter.
   • Destination MAC (6 bytes): Identifies the recipient.
   • Source MAC (6 bytes): Identifies the sender.
   • Type (2 bytes): Specifies encapsulated protocol (e.g., IPv4 = 0x0800).
2. Payload (46–1500 bytes): Encapsulates Layer 3 packet, padded if smaller than 46 bytes.
3. Trailer:
   • FCS (4 bytes): Frame Check Sequence for error detection.

MAC Addressing

1. Structure:
   • MAC addresses are 48-bit binary values, typically written in hexadecimal (e.g., 00:1A:2B:3C:4D:5E).
2. Types:
   • Burned-in Address (BIA): Permanent MAC address assigned during manufacture.
   • Unicast: Sent to a single device.
   • Broadcast: Sent to all devices on the LAN (FF:FF:FF:FF:FF:FF).
   • Multicast: Sent to a specific group of devices.

Key Ethernet Frame Types

• Unicast Frame: Sent to a single destination.
• Broadcast Frame: Sent to all devices in the LAN.
• Multicast Frame: Sent to a group of devices.

Examples

SOHO Network:

• A single switch connects PCs, a printer, and a router for WAN connectivity.

Enterprise Network:

• PCs on different floors communicate through access and distribution switches.

Frame Forwarding:

• A frame traverses from PC1 to PC2 via multiple switches and media types, with consistent Ethernet header and trailer.

Key Diagrams

• Figure 2-1: Typical SOHO Ethernet LAN.
• Figure 2-3: Enterprise LAN with hierarchical switching.
• Figure 2-9: Straight-through cabling pinout for 10/100 Mbps.
• Figure 2-15: Multimode fiber transmission.

Exam-Relevant Points

1. Cable Pinouts:
   • Straight-through: Connects unlike devices (PC ↔ Switch).
   • Crossover: Connects like devices (Switch ↔ Switch).
2. Auto-MDIX:
   • Automatically adjusts pinouts for mismatched cables.
3. MAC Addressing:
   • Unicast: Identifies a single device.
   • Broadcast: FFFF.FFFF.FFFF.
   • Multicast: Group-specific addresses.
4. Error Handling:
   • FCS detects but does not recover from errors.

Commands and Configurations

1. show commands:
   • show interfaces: Verifies interface speed and duplex.
   • show running-config: Checks auto-MDIX settings.
2. Configuration Tips:
   • Use full-duplex unless connected to a legacy hub.
   • Test cable pinouts with appropriate testers.

Common Issues and Troubleshooting

1. Cable Problems:
   • Incorrect straight-through or crossover usage.
   • Resolved by auto-MDIX on modern devices.
2. Duplex Mismatch:
   • Verify duplex settings to avoid performance degradation.
3. Frame Errors:
   • Use FCS for error detection; check physical layer for interference.

Advanced Topics

1. CSMA/CD (Carrier Sense Multiple Access with Collision Detection):
   • Used in half-duplex environments to handle collisions.
2. Full Duplex vs. Half Duplex:
   • Full duplex allows simultaneous send/receive.
   • Half duplex is required for hubs to avoid collisions.

Summary & Takeaways

• Ethernet is the foundation of wired LANs, with flexible standards for various environments.
• Proper cabling, frame forwarding, and error detection mechanisms ensure efficient network operations.
• Understanding duplex settings, pinouts, and Ethernet frame structure is crucial for CCNA success.